Operative relief of left ventricular outflow obstruction improves symptoms in most, but not all, patients with hypertrophic cardiomyopathy. To investigate whether pre-operative measurement of coronary flow reserve predict efficacy, twenty patients underwent study of coronary flow and lactate and oxygen metabolism within the myocardium, in the basal state and during rapid atrial pacing to a heart rate of 150. The six patients with flows more than 175 ml/min had significantly greater basal left ventricular outflow obstruction (90 plus or minus 30 vs. 50 plus or minus 32mmHg, p less than .01) compared to the remaining 14 patients. Severity of ischemia was similar in the two groups with 5 of 6 of the high flow patients and 8 of 14 of the remaining patients producing lactate during pacing stress. During post-operative study with relief of outflow obstruction, pacing stress resulted in substantially lower flows (216 plus or minus 18 pre-op to 151 plus or minus 46 ml/min post-op, p less than .05) in the 6 pre-op high flow patients compared to a lesser reduction in flow in the other 14 patients (138 plus or minus 19 preop to 123 plus or minus 43 ml/min postop, NS). Further, there was a substantial improvement in lactate metabolism in these 6 patients (28.6 plus or minus 23.9 mM x ml/min postoperative, versus -32.8 plus or minus 30.5 mM x ml/min preoperatively, p less than .05) with no patient producing lactate. In contrast, there was no net change overall in lactate metabolism in the Group B patients. Further, all 6 high flow patients had a 10 beat/min or greater improvement in pacing angina threshold, whereas 5 of the 14 low flow patients had no improvement. Thus, a higher peak flow response to stress consistently predicts postoperative improvement to pacing stress. Results in patients with lower peak flow responses are variable, suggesting mechanisms other than obstruction (small vessel disease, myocardial fibrosis, abnormal diastolic function, which might contribute to symptoms and be less likely to be benefited by relief of left ventricular outflow obstruction.